[REV] stock: revert of f229d40fb7a2a16c6f211a059694b18124b9719a

Reintroduction of field mes_type on product.template. This field was not
used in the code anymore, but we keep it for future use.

This field allows the user to express that there is no fixed conversion
rate between uom and uos.

(also very partial revert of 52c3b52bf09fea7e0920ebcec3f0e8acb066b2b7)